1. Field of the Invention
The present invention relates to a rotation angle detection apparatus and particularly relates to an apparatus, such as a resin rotary encoder, provided with a rotary disk. The invention also relates to a rotary disk used in such an apparatus.
2. Related Background Art
Many of the conventional rotary encoders are optical rotary encoders that detect rotation angle information optically or magnetic rotary encoders that detect rotation angle information magnetically. The optical rotary encoder is either a transmissive rotary encoder in which a light flux is adapted to be transmitted by a rotation angle information record part or a reflective rotary encoder in which a light flux is adapted to be reflected by a rotation angle information record part.
FIG. 11 shows, by way of example, a conventional transmissive rotary encoder. In this rotary encoder, a motor 1 supports a rotary shaft 2 by bearings 3 and 4. An attachment hub 5 made of a metal is fitted to the upper portion of the rotary shaft 2 and secured to the rotary shaft 2 by a set screw 6.
A rotary scale 7 serving as a rotation angle record part is mounted on the top surface of the attachment hub 5. The rotary scale 7 is secured to the attachment hub 5 by means of a setting ring 8 and adhesive 9 fixed to the rotary shaft 2. On the top surface of the motor 1, there is provided a transmissive type sensor head 11 with an electric circuit board 10 between.
The sensor head 11 has a frame 12 provided on the electric circuit board 10. The peripheral portion of the rotary scale 7 is disposed between an upper frame portion 12a and a lower frame portion 12b of the frame 12 in such a way that the peripheral portion of the rotary scale 7 is not brought into contact with the upper and lower frame portions 12a and 12b. The upper frame portion 12a of the frame 12 accommodates a light emitting element 13 and a collimator lens 14 arranged in the mentioned order from top down, while the lower frame portion 12b of the frame 12 accommodates a fixed scale 15 and a light receiving element 16 arranged in the mentioned order from top down.
The attachment hub 5 is made of a material having good machinability such as brass or aluminum. The inner surface of a fitting hole 5a to which the rotary shaft 2 is to be fitted and a supporting surface 5b on which the rotary scale is to be mounted are machined with high precision. The rotary scale 7 is made of a thin glass plate or a thin metal plate, and etching is applied to it or a PET film for photoengraving is attached to it.
Upon assembling the rotary encoder, the attachment hub 5 is fitted to the rotary shaft 2 of the motor 1 and the attachment hub is positioned to a predetermined thrust height position, and then the set screw 6 is tightened. Next, the rotary scale 7 is placed on the top surface of the attachment hub 5, and then the rotary scale 7 is provisionally secure by a setting ring 8 attached to the rotary shaft 2. Then, adjustment for aligning the center of a recorded pattern of the rotation angle information record part 7a of the rotary scale 7 and the center of the rotary shaft 2 is performed. After that, the rotary shaft 2 and the setting ring 8 are fixed to each other by the adhesive 9, and the rotary scale 7 and the setting ring 8 are also fixed to each other by the adhesive 9, as shown in FIG. 12. Finally, the sensor head 11 is brought to the vicinity of the rotary scale 7 in such a way that the outer periphery of the rotary scale 7 is inserted between the upper frame portion 12a and the lower frame portion 12b of the frame 12, and the sensor head 11 is secured at an appropriate position in relation to the motor 1.
A divergent light flux emitted from the light emitting element 13 of the sensor head 11 is transmitted through the collimator lens 14 so as to be converted into a substantially parallel light flux. This light flux passes through the rotation angle information record part 7a of the rotary scale 7 and then passes through the fixed scale 15 to enter the light receiving element 16. The sensor head 11 optically reads a change in moiré fringes caused by a change in relative angular positional relationship between the rotation angle information record part 7a and the fixed scale 15 in terms of a change in the quantity of light incident on the light receiving element 16 to detect the rotation angle of the rotary scale 7.
FIG. 13 shows a conventional reflective rotary encoder. This reflective rotary encoder has instead of the above-described sensor head 11 a sensor head 17 disposed beneath the rotary scale 7 for detecting a reflected light flux from a rotary angle information record part 7a of a rotary scale 7.
As will be understood from the above, manufacturing of a conventional rotary encoder involves many parts such as an attachment hub 5, a set screw 6, a setting ring 8 and adhesive 9. In addition, the adhesive 9 is applied to many portions, and a centering process is required to be performed. Thus, manufacturing of a conventional rotary encoder involves many processes and it is difficult to reduce the manufacturing cost. In addition, since the rotary scale 7 is formed on a thin glass plate or a thin metal plate by etching or as a photoengraving PET film attached thereto, it is impossible to attain a high degree of precision in coaxiality or in fitness.
Recently, it has been known in the field of rotary encoder production, to mold an attachment hub 5 and a rotary scale 7 integrally with a synthetic resin material and to mount the molded part in the form of a rotary disk to a rotation shaft 2 easily so as to reduce the manufacturing cost. For example, the applicant of this application has disclosed a rotary scale having a V-grooved grating in, for example, Japanese Patent Application Laid-Open No. 60-140119, Japanese Patent Application Laid-Open No. 62-3617, Japanese Utility Model Application Laid-Open No. 5-84818, Japanese Patent Application Laid-Open Nos. 5-39410 and 5-39411 and proposed a rotary scale having a cylindrical grating in for example Japanese Patent publication Nos. 2810521 and 2862417.
FIG. 14 shows a rotary encoder provided with a rotary disk 18 that has been integrally molded using a synthetic resin material. The rotary disk 18 is provided with an rotation angle information record part 18a. The rotary disk 18 is fitted to a rotary shaft 2 of a motor 1 that is similar to those described above, and the rotary shaft 2 and the rotary disk 18 are bonded by adhesive 19.
In this rotary encoder, the above-described attachment hub 5, the set screw 6, the setting ring 8 are not necessary. In addition, it is easy to attain required degree of precision in fitness of the rotary shaft 2 with the fitting hole 18b of the rotary disk 18 and in the coaxiality of the pattern of the rotation angle information record part 18a and the fitting hole 18b of the rotary disk 18. Furthermore, the above-mentioned centering process, which is the most troublesome process, can be omitted. Therefore, the manufacturing cost would be greatly reduced by virtue of the reduction of the number of the parts and the omission of the centering process.
However, in the case in which the rotary shaft 2 is made of a metal and the rotary disk 18 is made of a synthetic resin, when the ambient temperature greatly changes, the size of the rotary shaft 2 and the size of the rotary disk 18 will change in accordance with their respective thermal expansion coefficients, so that the relative position of the rotary shaft 2 and the rotary disk 18 will change, since the thermal expansion coefficient of a synthetic resin material is larger than the thermal expansion coefficient of a metal or a glass. Similarly, in the case in which the sensor head 11 is made of a synthetic resin, the sensor head can deviate from its optimum position due to its proper thermal characteristic coefficient.
Especially, when the dimensions of the rotary disk 18 and the sensor head 11 change with respect to the thrust direction of the rotary shaft 2, the optimum spacing between the rotation angle information record part 18a and the sensor head varies, so that the output signal of the sensor head also varies. Thus, the detection accuracy of the sensor head would be deteriorated. In the worst case, the rotary disk 18 and the sensor head 11 collide physically with each other to cause a damage.
In order to solve the above-described problem, it is necessary to reduce the mounting tolerance of the rotary disk 18 to the sensor head 11 with respect to the thrust direction as small as possible, which involves an increase in the manufacturing cost.